A federal judge struck down a California ban on same-sex marriages on Wednesday as unconstitutional, handing a key victory to gay rights advocates in a politically charged decision almost certain to reach the US Supreme Court.

Legal scholars said the decision has wide implications for nearly 40 states with similar laws on their books, making it more difficult to defend those measures in court on the basis of moral grounds or social tradition.

Still, US District Court Chief Judge Vaughn Walker ordered the voter-approved ban, known as Proposition 8, to remain in place at least temporarily until he decides on a request by supporters of the ban to keep it intact while the case moves to a higher court.

Although the result leaves gay men and lesbian couples unable to marry for now, Walker said Prop 8 opponents "demonstrated by overwhelming evidence" that it violates due process and equal-protection rights under the US Constitution.

"Proposition 8 fails to advance any rational basis in singling out gay men and lesbians for denial of a marriage licence," Walker wrote in the conclusion of his 136-page opinion.

Supporters of Prop 8, which California voters passed 52.5 per cent to 47.5 per cent in November 2008.